Method of stem construction for vacuum tubes



May 25, 1943. H L THORSON METHOD OF STEM CONSTRUCTION FOR VACUUM TUBES Filed Jan. 21, .1942

Patented May 25, 1943 METHOD OF STEM CONSTRUCTION FOR VACUUM TUBES Harry L. Thorson, Schenectady, N. Y assignor to General Electric Company, a corporation of New York Application January 21, 1942, Serial No. 427,606

Claims. ((31. 250- 275) The present invention relates-to lead-in seals for enclosed electrical apparatus such as evacuated electronic discharge devices, and more particularly to methods of making such seals.

The invention is principally concerned with the provision of an improved stem press of the type in which the lead-in wires are sealed in a circular arrangement through a unitary structure which includes a glass flare centrally joined to an exhaust tubulation. An important aspect of the invention consists in a' procedure by which a stem structure of this type may be built up from its elementary parts in a form which assures maximum protection of the stem seals from stresses apt to be encountered in the practical use of the device with which the stem is associated. a

The features of the invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the drawing in which Fig. 1 is a longitudinal sectional view of an electronic tube suitably embodying the invention; Fig. 2 is a sectional view which illustrates the prior art practice with respect to forming a stem press of the general character of that employed in the structure of Fig. 1; and Figs. 3, 4 and 5 illustrate successive steps in the improved procedure of the present invention. 7

Referring particularly to Fig. '1, there is shown an evacuated discharge tube of the type to which the present invention is applicable. This comprises an elongated cylindrical metal anode H! which constitutes a portion of the enclosing envelope for the tube. Within the anode and arranged coaxially therewith there is provided a heatable cathode comprising a plurality of filaments H which are in a hollow configuration. These filaments are in turn surrounded by a grid structure consisting of wires I2 wound helically on a plurality of stay rods l3 which parallel the filaments ll. 1

The open end of the anode I is hermetically sealed. to a glass enclosure l5, the sealed joint being indicated at I6. Within the part l there is provided a re-entrant circular press l8 which serves as a mount for a plurality of cathode leadin conductors l9.

These conductors, which may consist, for example, of tungsten, are arranged in circular array and are connected outside the enclosure with a plurality of current supply conductors 2|.

Within the enclosure the wires 19 are maintained in parallelism with one another by means of a pair of insulating disks 24 and 25 through which they pass. The individual wires are relatively stiff and provide fixed anchorages for the cathode filaments II to which they are joined at their extremities.

The ends of the filaments I l more remote from the supporting wires l9 are joined to a common terminal structure (not shown). This struc- I ture is fixedly secured to a rod 21 which is arranged at the center of the filament array and which serves as a means for applying tension to the filament. For furtherance of this latter object, the rod is associated with a compression spring (not shown) which is located within a two-parthousing'29 and which acts on the rod 21 to cause it to move in a longitudinal direction.

For the purpose of supporting the grid structure of the device, there is provided in the region surrounding the circular stem press iii a glass sleeve 3| which extends toward the interior of the discharge space. At its inner edge this sleeve is joined to a cylindrical metal member 32 which in turn is secured to a similarly formed member 33 placed in abutting relation with it. This latter member is connected directly to the grid stay rods l3. A terminal for the grid structure -is provided by means of a ring-like conductor 35 which is clamped between the members 32 and which connects with a lead-in wire 36. The wire 36 terminates in a cup-shaped contact member 38 which is sealed intothe Wall of the glass enclosure I5.

The present invention is primarily concerned with the procedure employed for fabricating the circular press l8 and in particular with a novel method of uniting it to the fiare 40 and the exhaust tubulation M of Fig. 1 in a manner which results in the production of a lead-in press having maximum protection from cracking.

The circular stem press is, in itself, well known to the art and has been conventionally produced in the manner indicated in Fig. 2. In the construction there shown a plurality of lead-in conductors are initially interposed between the outer surface of a cylindrical tribulation-bearing part 5| and the inner surface of the-throat of a flare part 53 which is so shaped as to fit loosely around the part 5i. With the various elements arranged in the relationship shown, the throat portion of the flare 53 is heated and subjected to pressure in such manner as to cause it to shrinl;

upon the part thus causing the conductors 59 to be embedded in the molten glass.

Seals fabricated in the manner specified have been successfully used in many connections. However, in certain applications, dilliculties arise from their use because of fracture of the glassto-metal seals existing between the glass part of the circular press and the conductors 50. Such cracking ordinarily occurs at the point B and is a result of weakness of the joint between the lead-in wire and the contiguous glass. Because of this weakness, it frequently happens that small cracks initiated at the region B develop through the glass wall and destroy the vacuum in the device with which the stem is associated.

The difficulties referred to in the foregoing are substantially wholly overcome by the use of the fabricating procedure which is illustrated in Figs. 3, 4 and 5. In this procedure an enlarged annular glass part 51 is first formed upon the extremity of an evacuating tubulation 58. Next, a glass sleeve 66 is placed loosely around the part 5'! with a number of lead-in conductors 62 interposed between them. (The conductors may at this time be connected to electrode elements or such elements may be attached later.) Thereafter the part 60 is heated to its softening temperature (e. g. by means of torches 84) and suflicient mechanical pressure is exerted on the outer surface of the sleeve (as by means of a contractible clamping ring) to cause it to be compressed against the part 51 in such manner as hermetically to seal to the conductors 62.

The completed form of a seal produced in this way is illustrated in Fig. 4. As clearly appears from this figure, both extremities of the seal are readily accessible so that these extremities and the entire seal structure can be uniformly subjected toheat (as by the torches 64) in such as way as to cause the glass to flow around and into good sealing contact with the lead-in conductors. Accordingly, and contrary to the condition which exists in connection with the structure of Fig. 2, there are no weak joints at which cracks can develop.

After the seal is formed in the manner specified, it may be joined to a suitable flare (e. g. the flare 61 of Fig. 5) by fusing the inner edge of the fiare to the outer surface of the sleeve 68. In the preferred arrangement, the fiare is secured to the central portion of the sleeve since this results in a symmetrical arrangement which tends to minimize the creation of strains as a result of the sealing on of the flare. Moreover, this arrangement assures that in the event the externally extending parts of the lead-in conductors shall accidentally be subjected to forces of such magnitude as to injure the seals, any cracks initiated in the sleeve 60 (i. e., in its lower half) will be at a point remote from the vacuum enclosure. However, it is desirable in some cases for constructional reasons to join the fiare to another part of the sleeve Ell, as, for example, by affixing it at the point C (Fig. 5). In this case the completed structure has substantially the appearance of that of Fig. 2. but the glass-to-metal seals are of superior quality because of the difference in the manner of their production.

It will be understood, of course, that in using the structure of Fig. 5 it is first annealed and then sealed into combination with the other elements of a complete discharge envelope, for example, in a relationship such as that shown in Fig. 1. The tubulation depending from the part BI is used for evacuating the envelope a d is itself finally sealed off.

While the invention has been described with reference to a particular application, it will be understood that numerous other applications will occur to those skilled in the art. I, therefore, aim in the appended claims to cover all such uses of the invention as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of forming a lead-in stem which comprises encircling a central part with a loosely fitting glass sleeve, positioning one or more lead-in conductors in the space between said central part and the sleeve, fusing the sleeve into engagement with the lead-in conductors and the said central part, and thereafter joining the inner edge of a separately formed flare to an externally accessible surface of the sleeve.

2. The method of forming a lead-in stem '01" a type which in finished form includes an evacuating tubulation joined to a flare, which method comprises encircling a tubulation-bearing part with a loosely fitting glass sleeve, positioning one or more lead-in conductors in the space between said tubulation-bearing part and said sleeve, fusing the sleeve into engagement with the tubulation-bearing part and the lead-in conductors, and thereafter joining the inner edge of a separately formed fiare to an externally accessible surface of the said sleeve.

3. The method of fabricating a lead-in stem of a type which in finished form includes an evacuating tubulation joined to a flare, which method comprises initially forming on one end of the tubulation an enlarged annular glass part, encircling said part with a loosely fitting glass sleeve, positioning a lead-in conductor in the space between said annular part and said sleeve, fusing the sleeve into engagement with the annular part, and thereafter joining the inner edge of a separately formed flare to said sleeve.

4. The method of fabricating a lead-in stem which comprises forming an evacuating tubulation having an enlarged annular part on one end thereof, encircling the said part with a loosely fitting glass sleeve, positioning one or more lead-in conductors in the space between said annular part and said sleeve, fusing the sleeve into sealing engagement with the lead-in conductors and the annular part, heating both extremities of the circular press thus formed to improve the seals produced between the glass and the lead-in conductors, and thereafter joining the inner edge of a separately formed 'fiare to the exposed surface of the said sleeve.

HARRY L. THORSON. 

